How to re-link files which have paths that have become invalid (can't be found)?
To re-link missing files, open file menu, then choose External Data -> Find Missing Files, and select the folder where the image textures are located. File names have to be the same as they are in the blend file.

The files are found based on matching names, the folder is searched recursively, so you can for example point it at the base of your project path, to perform a more extensive search.